fix learning path count, increase item page size

[ChristopherChudzicki]

What are the relevant tickets?

• Closes https://github.com/mitodl/hq/issues/4737

Description (What does it do?)

Increases learning path page size to 100 and fixes the item count.

How can this be tested?

1. Create a learning path with more than 10 items. (But less than 100!).
   • as a staff member, use the left "list" button on search page cards.
2. View that learning path at /learningpaths/... click into it for the "Details" view.
   • The "Items " should accurately reflect the number of items. (Before it was capped at the page size).
   • All the items should display.